1. Field of the Invention
This invention relates to a system and method for separating nitrogen from methane and other components of natural gas streams being processed into a sales gas stream and a natural gas liquids (NGL) stream by the known gas subcooled/expander process (GSP/expander process or simply GSP). The system and method of the invention are particularly suitable for use in connection with natural gas streams where the processed sales gas stream contains nitrogen in an amount that exceeds pipeline specifications.
2. Description of Related Art
Nitrogen contamination is a frequently encountered problem in the production of natural gas. Transporting pipelines typically do not accept natural gas containing more than about 4 mole percent inerts, such as nitrogen. Estimates indicate that as much as 25% of natural gas in the United States exceeds a typical 4% pipeline specification. To correct this problem, the sales gas may be mixed or diluted with other gas to achieve the desired nitrogen specification. Alternatively, known methods of nitrogen removal such as a nitrogen rejection unit or NRU comprised of two cryogenic fractionating columns, as described in U.S. Pat. Nos. 4,451,275 and 4,609,390 or comprised of a single fractionating column, as described in U.S. Pat. Nos. 5,141,544, 5,257,505, and 5,375,422 may be used. However, dilution and full-blown NRU installation and operation are expensive for the gas processor. Additionally, a complete stand-alone NRU, which is capable of removing large percentages of nitrogen, may not be necessary or economically feasible for a gas subcooled process where the sales gas exceeds the nitrogen specification by only a small amount.